CHANGING THE LIGHT-INTENSITY OF THE VISUAL ENVIRONMENT RESULTS IN LARGE DIFFERENCES IN NUMBERS OF SYNAPSES AND IN PHOTORECEPTOR SIZE IN THERETINA OF THE YOUNG-ADULT RAT

A quantitative light- and electron-microscopic study has been made of the retinae of rats which were exposed to different lighting condition s for between one and 15 weeks in young adulthood, having been reared in identical conditions during development. The width of the inner and outer segments of the photoreceptors and the width of the outer plexi form layer varied inversely with the light intensity under diurnal lig hting conditions of 10 h light/14 h dark. Linear regression analysis s howed that the widths were inversely related to the fourth root of the light intensity as measured in lux. Both central and peripheral areas of retina showed a similar change. No change was seen in the widths o f the inner plexiform layer, or of the inner and outer nuclear cell la yers. Nor was there a difference in the packing density or size of the nuclei in the nuclear cell layers. The number of ribbon synapses in t he outer plexiform layer also varied inversely with the intensity of d iurnal light. Linear regression analysis showed that the number of syn apses was inversely correlated with the fourth root of the light inten sity and was positively correlated with the width of the outer plexifo rm layer. The number of ribbon synapses was increased by up to two and a half times in constant darkness compared to diurnal light of 35 lux . The increase was present but not maximal after one week of exposure. The length of synaptic ribbons was unchanged. The nerve terminals for ming such synapses were increased in size but not in number. After one week, there was little or no additional change in the retinal widths and number of synaptic ribbons with time. However, there was a progres sive increase with time in nerve terminal size (two-fold in area) in c onstant darkness. There was some evidence of a slight decrease in nerv e terminal number and increase in size of retinal nuclei with age. It is concluded that the adult retina responds to a different lighting en vironment by a relatively rapid change in the size of photoreceptor se gments, by a progressive and large change in number of ribbon synapses and by a slower progressive and large change in the size of photorece ptor nerve terminals. The response is quantitatively determined by the strength of the stimulus but not in a linear fashion. These results a re compared with the effects of environmental stimulation of other are as of the nervous system.